South Korea is closely monitoring North Korea's military moves in coordination with the United States, the Joint Chiefs of Staff (JCS) said Monday, amid signs of activity at the North's main shipyard used to develop submarine-launched ballistic missiles (SLBMs).

According to 38 North, commercial satellite imagery of the North's Sinpo Shipyard indicates that a missile canister for the submarine missile test barge has likely been removed, possibly for maintenance or for the replacement of a new canister or launch frame to accommodate larger SLBMs.

"Our military has been closely tracking and monitoring North Korea's military moves, while the South Korean and the US intelligence authorities have been maintaining close cooperation," JCS spokesperson Col. Kim Jun-rak told a regular briefing.

"Keeping various possibilities in mind, we maintain a readiness posture," he added.

Asked about any signs that North Korea is ready to launch a new 3,000-ton submarine, the spokesperson said the authorities are watching related moves closely, leaving every possibility open.

Sources said earlier that the communist country is believed to have completed the construction of the new submarine, which is expected to be capable of carrying three SLBMs. Pyongyang first unveiled the asset in July 2019. (Yonhap)